Trapping System.NullReferenceException on start-up

26 Oct 2006 11:05 AM

Mark Raishbrook

A client of mine is getting a NullReferenceException when starting up a small
app I've written. I've traced the cause to an unregistered DLL the app
references, so the error itself is not the problem. What I would like to do
is trap it, but I'm not sure where I should do this. Here's an excerpt from
the exception text:

System.NullReferenceException: Object reference not set to an instance of an
object.
at HDi.Form1.Form1_Load(Object sender, EventArgs e)
at System.EventHandler.Invoke(Object sender, EventArgs e)
at System.Windows.Forms.Form.OnLoad(EventArgs e)
[snip]

So my question is: where can I trap this error? Is there a way to test for
the presence of a DLL (and to check it's registered) before the start-up form
is created and this error is generated? Any help would be greatly appreciated.

26 Oct 2006 12:18 PM

Norman Chong

Mark Raishbrook schrieb:

>
> So my question is: where can I trap this error? Is there a way to test for
> the presence of a DLL (and to check it's registered) before the start-up form
> is created and this error is generated? Any help would be greatly appreciated.
>

Hi Mark,

If you want to do something before your forms are created, you can do
this in the 'Startup'-Event in class ApplicationEvents.vb, which shows
up when you click on 'Application' => 'View Application Events' in your
project file. But I don't know if this really solves your problem, as
I'm not sure if those event is called before creating the references...
Somehow I don't think so.
So maybe it's best when you load your dll dynamically where you need
it.
Here's the code for loading a .NET assembly ( I don't know if it's the
same when loading a COM )

Public Class LoadAssembly
   Private myAssembly as Reflection.Assembly

   Sub New()
      ' Load the assembly and handle errors ( e.g. .dll doesn't exist )
      Try
         myAssembly =
Reflection.Assembly.LoadFile(<CompletePathToAssembly>)
      Catch ex As Exception
         ' Errorhandling
      End Try

      ' Create an instance of a class from the loaded assembly
      Dim tTypes() As Type = myAssembly.GetTypes()
      Dim tType As Type
      Dim myAssemblyObject as Object
      For Each tType In tTypes
         If (tType.Name.Equals(<NameOfClassYouWantToLoad>)) Then
            myAssemblyObject = Activator.CreateInstance(tType, args)
         End If
      Next
   End Sub
End Class

Hope this helps :-)

26 Oct 2006 2:39 PM

Mark Raishbrook

Many thanks for that, Norman. I'll give it a try and see how it goes.

Show quoteHide quote

"Norman Chong" wrote:

> > So my question is: where can I trap this error? Is there a way to test for
> > the presence of a DLL (and to check it's registered) before the start-up form
> > is created and this error is generated? Any help would be greatly appreciated.

> Hi Mark,
>
> If you want to do something before your forms are created, you can do
> this in the 'Startup'-Event in class ApplicationEvents.vb, which shows
> up when you click on 'Application' => 'View Application Events' in your
> project file. But I don't know if this really solves your problem, as
> I'm not sure if those event is called before creating the references...
> Somehow I don't think so.
> So maybe it's best when you load your dll dynamically where you need
> it.
> Here's the code for loading a .NET assembly ( I don't know if it's the
> same when loading a COM )
>
> Public Class LoadAssembly
>    Private myAssembly as Reflection.Assembly
>
>    Sub New()
>       ' Load the assembly and handle errors ( e.g. .dll doesn't exist )
>       Try
>          myAssembly =
> Reflection.Assembly.LoadFile(<CompletePathToAssembly>)
>       Catch ex As Exception
>          ' Errorhandling
>       End Try
>
>       ' Create an instance of a class from the loaded assembly
>       Dim tTypes() As Type = myAssembly.GetTypes()
>       Dim tType As Type
>       Dim myAssemblyObject as Object
>       For Each tType In tTypes
>          If (tType.Name.Equals(<NameOfClassYouWantToLoad>)) Then
>             myAssemblyObject = Activator.CreateInstance(tType, args)
>          End If
>       Next
>    End Sub
> End Class
>
> Hope this helps :-)